Population Structure of Geosmithia morbida, the Causal Agent of Thousand Cankers Disease of Walnut Trees in the United States PublicDeposited

To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The article was published by the Public Library of Science and is in the public domain. The published article can be found at: http://www.plosone.org/.

Descriptions

The ascomycete Geosmithia morbida and the walnut twig beetle Pityophthorus juglandis are associated with thousand
cankers disease of Juglans (walnut) and Pterocarya (wingnut). The disease was first reported in the western United States
(USA) on several Juglans species, but has been found more recently in the eastern USA in the native range of the highly
susceptible Juglans nigra. We performed a comprehensive population genetic study of 209 G. morbida isolates collected
from Juglans and Pterocarya from 17 geographic regions distributed across 12 U.S. states. The study was based on sequence
typing of 27 single nucleotide polymorphisms from three genomic regions and genotyping with ten microsatellite primer
pairs. Using multilocus sequence-typing data, 197 G. morbida isolates were placed into one of 57 haplotypes. In some
instances, multiple haplotypes were recovered from isolates collected on the same tree. Twenty-four of the haplotypes
(42%) were recovered from more than one isolate; the two most frequently occurring haplotypes (H02 and H03)
represented 36% of all isolates. These two haplotypes were abundant in California, but were not recovered from Arizona or
New Mexico. G. morbida population structure was best explained by four genetically distinct groups that clustered into
three geographic regions. Most of the haplotypes isolated from the native range of J. major (Arizona and New Mexico) were
found in those states only or present in distinct genetic clusters. There was no evidence of sexual reproduction or genetic
recombination in any population. The scattered distribution of the genetic clusters indicated that G. morbida was likely
disseminated to different regions at several times and from several sources. The large number of haplotypes observed and
the genetic complexity of G. morbida indicate that it evolved in association with at least one Juglans spp. and the walnut
twig beetle long before the first reports of the disease.